This invention relates to a trigger arrangement comprising a series-combination of a first and a second bistable element, an input of each of which is coupled to a trigger input for a trigger input signal, the trigger arrangement being provided with a release input for a release signal, which is coupled to at least one of the bistable elements, and with a trigger output for a trigger output signal, which assumes a first stable state when the release signal assumes a first value and which assumes a second stable state, which state is determined by the trigger input signal after the release signal has assumed a second value, and which state is assumed when an output of the first bistable element assumes a second stable state after the trigger signal exceeds a first level, whereupon an output of the second bistable element assumes a second stable state after the trigger input signal falls below a second level, which is lower than the first level.
Such a trigger arrangement can generally be used for producing trigger signals from a periodical signal at a predetermined level and at predetermined instants. Such a trigger arrangement is particularly suitable for use in oscilloscopes comprising as the display device a cathode-ray tube to permit operation up to high frequencies of the signal without the signal displayed by the display device moving (a so-called "jitter"-free image).
Such an arrangement is known from U.S. Pat. No. 3,649,852, in which a trigger arrangement is described especially intended to be used in oscilloscopes. The known arrangement comprises the said series-connected bistable elements to each of which the trigger input is coupled. Further, the arrangement comprises the release input, which is driven, for example, by a saw-tooth generator for producing a time base of an oscilloscope. As long as the release signal at the release input occurs with the first value, the trigger output has the first stable state. When the release signal then assumes the second value, the trigger output does not immediately respond thereto, but responds only after the output of the first bistable element has first assumed the second stable state after the trigger input signal has exceeded the first level and then the output of the second bistable element has assumed the second state after the trigger input signal falls below the second level. If the last-mentioned situation occurs, the trigger output assumes the second state. The saw-tooth generator to be driven, for example, by the trigger arrangement is started at the transition of the trigger output from the first state to the second state. The arrangement, of which the bistable elements are constituted, for example, by OR gates fed back via resistors and comprising emitter-coupled transistor logic elements, operates satisfactorily up to high frequencies. However, the arrangement no longer operates satisfactorily at very high frequencies because if the release signal passes from the first value to the second value immediately before the instant at which the trigger input signal exceeds the first level, the second bistable element is released at practically the same instant due to finite rise times of logic signals so that the instant of change also practically coincides therewith, which results in an uncertainty in the occurrence of the change at the trigger output with respect to the instant at which the second bistable element is released. Further, the instant at which, after release of the arrangement, the trigger output assumes the second state is fairly dependent upon the level difference between the first and the second level and upon the amplitude of the trigger input signal. In practical situations, it is moreover desirable that the amplitude of the trigger input signal be large with respect to, for example, the first and the second level, as a result of which the uncertainty increases even further.